Enhancement of Molecular Sieve Performance

ABSTRACT

A catalyst for converting methanol to light olefins and the process for making and using the catalyst are disclosed and claimed. SAPO-34 is a specific catalyst that benefits from its preparation in accordance with this invention. A seed material is used in making the catalyst that has a higher content of the EL metal than is found in the principal part of the catalyst. The molecular sieve has predominantly a roughly rectangular parallelepiped morphology crystal structure with a lower fault density and a better selectivity for light olefins.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.11/171,801 filed Jun. 30, 2005, the contents of which are herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a process for producing an enhanced catalyst,the catalyst and the use of this catalyst for converting oxygenates tolight olefins. More particularly, the process of this invention ishighly efficient in converting methanol to light olefins due to the useof catalysts having a favorable catalyst.

BACKGROUND OF THE INVENTION

Olefins are traditionally produced from petroleum feedstock by catalyticor steam cracking processes. These cracking processes, especially steamcracking, produce light olefin(s) such as ethylene and/or propylene froma variety of hydrocarbon feedstocks. It has been known for some timethat oxygenates, especially alcohols, e.g. methanol, are convertibleinto light olefin(s). The preferred methanol conversion process isgenerally referred to as methanol-to-olefin(s) (MTO) process, wheremethanol is converted to ethylene and propylene in the presence of amolecular sieve.

The limited supply and increasing cost of crude oil has prompted thesearch for alternative processes for producing hydrocarbon products. Animportant type of alternate feed for the production of light olefins areoxygenates, such as alcohols, particularly methanol and ethanol, etherssuch as dimethyl ether, methyl ethyl ether, and diethyl ether, dimethylcarbonate, and methyl formate. These oxygenates may be produced byfermentation, or from synthesis gas derived from natural gas, petroleumliquids, carbonaceous materials, including coal, recycled plastics,municipal wastes, or other organic materials. One process that isparticularly useful in producing olefins is the conversion of methanolto hydrocarbons and especially to light olefins. The commercial interestin the MTO process is based on the fact that methanol can be obtainedfrom readily available raw materials such as coal or natural gas whichare treated to produce synthesis gas which is in turn processed toproduce methanol.

Oxygenates are converted to an olefin product through a catalyticprocess. The conversion of a feed containing oxygenates is usuallyconducted in the presence of a molecular sieve catalyst. AlthoughZSM-type molecular sieves and other molecular sieves may be used for theproduction of olefins from oxygenates, silicoaluminophosphate (SAPO)molecular sieves have been found to be of particular value in thiscatalytic process.

Silicoaluminophosphate molecular sieves are manufactured from sources ofsilicon, such as a silica sol, aluminum, such as hydrated aluminum oxideand phosphorus, such as orthophosphoric acid. In addition, an organictemplate such as tetraethylammonium hydroxide, isopropylamine ordi-n-propylamine is used. SAPO-34 belongs to the family of molecularsieves having the structure type of the zeolitic mineral chabazite(CHA). The CHA framework type has a double six-ring structure in an ABCstacking arrangement when viewed perpendicular to the rhombohedral3-fold axis.

The preparation and characterization of SAPO-34 has been reported inseveral patents including U.S. Pat. No. 4,440,871 and U.S. Pat. No.5,248,647, both of which are herein fully incorporated by reference.

One of the most important embodiments of the MTO conversion process isdirected to the production of light olefins, i.e., olefins containingfrom 2 to 4 carbon atoms, inclusive. Accordingly, it is important toutilize a catalyst which maximizes the production of these products,results in a high degree of conversion of the starting methanol, anddoes not deactivate rapidly under the process conditions imposed. In theconversion of methanol to olefins, SAPO-34 exhibits relatively highproduct selectivity to ethylene and propylene, and low productselectivity to paraffin and olefin with four or more carbons (C₄+olefin).

The effect of the particle size of the molecular sieve on activity hasalso been documented in U.S. Pat. No. 5,126,308. In the '308 patent, itis disclosed that molecular sieves in which 50% of the molecular sieveparticles have a particle size less than 1.0 μm and no more than 10% ofthe particles have a particle size greater than 2.0 μm have increasedactivity and/or durability. The '308 patent also discloses thatrestricting the silicon content to about 0.005 to about 0.05 molefraction also improves catalytic performance.

One desirable group of silicoaluminophosphate molecular sieves is thosethat have low silicon content. Silicoaluminophosphates of the CHAframework type with low silicon content are particularly desirable foruse in the MTO process. Low silicon content has the effect of reducingpropane formation and decreasing catalyst deactivation. However, it hasproven difficult to make pure phase CHA silicoaluminophosphate molecularsieves with low silica to alumina ratio.

In the art, various attempts have been made to improve the synthesis ofAlPO₄ or SAPO molecular sieves. One approach has been the addition of asource of fluoride ions to the synthesis mixture. However, this approachhas the disadvantage that many of the fluorides cause cost, safety orenvironmental concerns due to their toxicity, corrosiveness andvolatility. It would be highly desirable to have a process that avoidstheir use. U.S. Pat. No. 6,620,983 B1 describes the use of otherfluorine containing compounds. These compounds have two or more fluorinesubstituents, as the source of fluoride ion, in the synthesis ofaluminophosphates or silicoaluminophosphates. Although the molecularsieves produced are described as having the desired chabazite crystalstructure, they produce a lower than desired yield of light olefins whenused in a methanol to olefins process. Therefore, these other fluorinecontaining compounds are not the solution sought. It would also bedesirable to have a more effective catalyst in conversion of oxygenatesto olefins.

It is therefore desirable to find new processes, which are specific forthe synthesis of molecular sieves having the CHA framework type. Aparticular need is to find methods of preparing low silica SAPOmolecular sieves, which do not require the use of hydrogen fluoride orother fluorides.

SUMMARY OF THE INVENTION

The present invention provides a method for the synthesis ofmetalloaluminophosphate molecular sieves that overcomes many of theproblems inherent in the prior art methods of synthesis.

More specifically, the method for preparing metalloaluminophosphatemolecular sieves comprises first providing a seed containing from about5 to 20 wt-% metal oxide on a dry oxide basis and then forming areaction mixture comprising said seeds, a source of aluminum, a sourceof phosphorus, at least one organic template, and, a source of metal,and then inducing crystallization of metalloaluminophosphate molecularsieve from the reaction mixture. The source of metal provides a lowerconcentration of metal oxide to the reaction mixture than is present inthe seed. From 1-5 wt % metal oxide is present in these reactionmixtures. In a preferred embodiment of the invention, the metal issilicon and the metal oxide is SiO₂.

This invention also comprises using these silicoaluminophosphatemolecular sieves as catalysts that contain binders for convertingmethanol to light olefins.

The invention further comprises a novel crystallinesilicoaluminophosphate molecular sieve in which a first portion of themolecular sieve comprises a higher concentration of silica than does alarger second portion of the molecular sieve. In one embodiment of theinvention, the first portion of the molecular sieve comprises about 5-20wt-% silica and the larger second portion comprising the remainder ofthe molecular sieve comprises about 1-5 wt % silica. In the presentinvention, the preferred molecular sieves have a CHA framework type, AEIframework type, an intergrowth of CHA and AEI framework types or amixture of at least two of said framework types.

DETAILED DESCRIPTION OF THE INVENTION

The process of the instant invention concerns the preparation of and theuse of silicoaluminophosphate molecular sieves which is a type of ELAPOmolecular sieve. Other types of aluminophosphates may be prepared aswell. ELAPOs are molecular sieves which have a three-dimensionalmicroporous framework structure of AlO₄, PO₄ and ELO₄ tetrahedral units.Generally the ELAPOs have the empirical formula:

(EL_(x)Al_(y)P_(z))O₂

where EL is a metal selected from the group consisting of silicon,magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixturesthereof, “x” is the mole fraction of EL and has a value of at least0.005, “y” is the mole fraction of Al and has a value of at least 0.01,“z” is the mole fraction of P and has a value of at least 0.01 andx+y+z=1. When EL is a mixture of metals, “x” represents the total amountof the metal mixture present. Preferred metals (EL) are silicon,magnesium and cobalt with silicon being especially preferred. Themolecular sieves of the present invention are made with a seed materialof higher metal content than is present in the final molecular sieveproduct. The seed material comprises from 5 to 20 wt % metal andpreferably 8 to 15 wt % metal and most preferably 10 to 15 wt % metal. Alower level of metal content is found in the other ingredients that areused to prepare the molecular sieve product. These other ingredientscomprise from 1 to 5 wt % metal and preferably 2 to 4 wt % metal.

The preparation of various ELAPOs are well known in the art. Generally,ELAPO molecular sieves are synthesized by hydrothermal crystallizationfrom a reaction mixture containing reactive sources of EL, aluminum,phosphorus and a templating agent. Reactive sources of EL are the metalsalts such as the chloride and nitrate salts. When EL is silicon,sources of silicon include fumed, colloidal, precipitated or alkoxideforms of silica. Reactive sources of aluminum and phosphorus arepseudo-boehmite alumina and phosphoric acid. Templating agents that areused include amines and quaternary ammonium compounds. A frequently usedtemplating agent is tetraethylammonium hydroxide (TEAOH). When themolecular sieves are calcined, the resulting AlPO's or SAPO's have anx-ray diffraction (XRD) pattern typical of the CHA framework type andare of high purity in terms of their framework type. Seed crystalshaving higher silicon concentration than the desired molecular sieve areused to produce the CHA framework type crystals. Without the use of theseed crystals of higher silicon concentration than the remainder of thereaction mixture, the intergrowth content is higher.

The reaction mixture, consisting of a source of aluminum, a source ofphosphorous, one or more templating agents, seed crystals, and, one ormore metal containing compounds is placed in a sealed pressure vessel,optionally lined with an inert plastic material such aspolytetrafluoroethylene and heated preferably under autogenous pressureat a temperature between about 50° and 250° C. and preferably betweenabout 100° and 200° C. for a time sufficient to produce crystals.Typically, the time varies from about 1 to about 120 hours andpreferably from about 24 to about 48 hours. The desired product isrecovered by any convenient separation method such as centrifugation,filtration or decanting.

The molecular sieves of the present invention may be combined with oneor more formulating agents, to form a molecular sieve catalystcomposition or a formulated molecular sieve catalyst composition. Theformulating agents may be one or more materials selected from the groupconsisting of binding agents, matrix or filler materials catalyticallyactive materials and mixtures thereof. This formulated molecular sievecatalyst composition is formed into useful shape and sized particles bywell-known techniques such as spray drying, pelletizing, or extrusion.Matrix materials are typically effective in reducing overall catalystcost, in acting as thermal sinks that assist in shielding heat from thecatalyst composition, for example, during regeneration, densifying thecatalyst composition, increasing catalyst strength such as crushstrength and attrition resistance, and in controlling the rate ofconversion in a particular process.

Upon combining the molecular sieve and the matrix material, optionallywith a binder, in a liquid to form a slurry, mixing, preferably rigorousmixing, is needed to produce a substantially homogeneous mixturecontaining the molecular sieve. Examples of suitable liquids includewater, alcohol, ketones, aldehydes, esters and combinations thereof. Themost preferred liquid is water. The slurry may be colloid-milled for aperiod of time sufficient to produce the desired slurry texture,sub-particle size, and/or sub-particle size distribution.

The molecular sieve and matrix material, and the optional binder, may bein the same or different liquids, and may be combined in any order,either together, simultaneously, sequentially, or a combination thereof.In the preferred embodiment, the same liquid, preferably water is used.The molecular sieve, matrix material, and optional binder, are combinedin a liquid as solids, substantially dry or in a dried form, or asslurries, together or separately. If solids are added together as dry orsubstantially dried solids, it is preferable to add a limited and/orcontrolled amount of liquid.

In one embodiment, the slurry of the molecular sieve, binder and matrixmaterials is mixed or milled to achieve a sufficiently uniform slurry ofsmaller particles that is then fed to a forming unit to produce themolecular sieve catalyst composition. A spray dryer is often used as theforming unit. Typically, the forming unit is maintained at a temperaturesufficient to remove most of the liquid from the slurry, and from theresulting molecular sieve catalyst composition. The resulting catalystcomposition when formed in this way takes the form of microspheres.

Generally, the particle size of the powder is controlled to some extentby the solids content of the slurry. However, the particle size of thecatalyst composition and its spherical characteristics are alsocontrollable by varying the slurry feed properties and conditions ofatomization. Also, although spray dryers produce a broad distribution ofparticle sizes, classifiers are normally used to separate the fineswhich can then be milled to a fine powder and recycled to the spraydryer feed mixture.

After the molecular sieve catalyst composition is formed in asubstantially dry or dried state, a heat treatment such as calcination,at an elevated temperature is usually performed to further harden and/oractivate the formed catalyst composition. A conventional calcinationenvironment is air that typically includes a small amount of watervapor. Typical calcination temperatures are in the range from about 400°to about 1000° C., preferably from about 500° to about 800° C., and mostpreferably from about 550° to about 700° C. The calcination environmentis a gas such as air, nitrogen, helium, flue gas (combustion productlean in oxygen), or any combination thereof.

In one embodiment, calcination of the formulated molecular sievecatalyst composition is carried out in any number of well known devicesincluding rotary calciners, fluid bed calciners, batch ovens, and thelike. Calcination time is typically dependent on the desired degree ofhardening of the molecular sieve catalyst composition and thetemperature.

In one embodiment, the molecular sieve catalyst composition is heated innitrogen at a temperature of from about 600° to about 700° C. Heating iscarried out for a period of time typically from 30 minutes to 15 hours,preferably from 1 hour to about 10 hours, more preferably from about 1hour to about 5 hours, and most preferably from about 2 hours to about 4hours.

In addition to the molecular sieve of the present invention, thecatalyst compositions of the present invention may comprise one orseveral other catalytically active materials.

The molecular sieve catalysts and compositions of the present inventionare useful in a variety of processes including: cracking, hydrocracking,isomerization, polymerization, reforming, hydrogenation,dehydrogenation, dewaxing, hydrodewaxing, absorption, alkylation,transalkylation, dealkylation, hydrodecyclization, disproportionation,oligomerization, dehydrocyclization and combinations thereof.

The catalyst prepared in accordance with the present invention isparticularly useful in a process directed to the conversion of afeedstock comprising one or more oxygenates to one or more olefin(s).Preferably, the oxygenate in the feedstock comprises one or morealcohol(s), preferably aliphatic alcohol(s) where the aliphatic moietyof the alcohol(s) has from 1 to 20 carbon atoms, preferably from 1 to 10carbon atoms, and most preferably from 1 to 4 carbon atoms. The alcoholsuseful as feedstock in the process of the invention include lowerstraight and branched chain aliphatic alcohols and their unsaturatedcounterparts.

Non-limiting examples of oxygenates include methanol, ethanol,n-propanol, isopropanol, methyl ethyl ether, dimethyl ether, diethylether, di-isopropyl ether, formaldehyde, dimethyl carbonate, dimethylketone, acetic acid, and mixtures thereof. In the most preferredembodiment, the feedstock is selected from one or more of methanol,ethanol, dimethyl ether, diethyl ether or a combination thereof, morepreferably methanol and dimethyl ether, and most preferably methanol.

The feedstock, preferably comprising one or more oxygenates, isconverted in the presence of a molecular sieve catalyst composition intoone or more olefin(s) having 2 to 6 carbons atoms, preferably 2 to 4carbon atoms. Most preferably, the olefin(s), alone or combination, areconverted from a feedstock containing an oxygenate, preferably analcohol, most preferably methanol, to the preferred olefin(s) ethyleneand/or propylene.

The process of converting methanol to olefins is generally referred toas “MTO”. In an MTO process, an oxygenated feedstock, containingmethanol as a primary component of the feedstock, is converted in thepresence of a molecular sieve catalyst composition into one or moreolefin(s), predominantly, ethylene and/or propylene, often referred toas light olefin(s). The amount of light olefin(s) produced based on thetotal weight of hydrocarbon produced is at least 50 wt-%, preferablygreater than 60 wt-%, more preferably greater than 70 wt-%. Higheryields may be obtained through improvements in the operation of theprocess as known in the art.

The feedstock may contain at least one or more diluents, typically usedto reduce the concentration of the feedstock that is reactive toward themolecular sieve catalyst composition. Examples of diluents includehelium, argon, nitrogen, carbon monoxide, carbon dioxide, water,essentially non-reactive paraffins (especially alkanes such as methane,ethane, and propane), essentially non-reactive aromatic compounds, andmixtures thereof. The preferred diluents are water and nitrogen, withwater being particularly preferred. Water, can be used either in aliquid or a vapor form, or a combination thereof. The diluent is eitheradded directly to a feedstock entering into a reactor or added directlyinto a reactor, or added with a molecular sieve catalyst composition.The amount of diluent in the feedstock is generally in the range of fromabout 5 to about 50 mol-% based on the total number of moles of thefeedstock and diluent, and preferably from about 5 to about 25 mol-%.

The reaction processes can take place in a variety of catalytic reactorssuch as hybrid reactors that have a dense bed or fixed bed reactionzones and/or fast fluidized bed reaction zones coupled together,circulating fluidized bed reactors, riser reactors, and the like.

In the preferred embodiment, a fluidized bed process or high velocityfluidized bed process includes a reactor system, a regeneration systemand a product recovery system.

The fluidized bed reactor system has a first reaction zone within one ormore riser reactor(s) and a second reaction zone within at least onedisengaging vessel, preferably comprising one or more cyclones. Theriser reactor(s) and disengaging vessel are contained within a singlereactor vessel. Fresh feedstock is fed to the one or more riserreactor(s) in which a molecular sieve catalyst composition or cokedversion thereof is introduced. In one embodiment, the molecular sievecatalyst composition or coked version thereof is contacted with a liquidor gas, or combination thereof, prior to being introduced to the riserreactor(s), preferably the liquid is water or methanol, and the gas isan inert gas such as nitrogen. If regeneration is required, thesilicoaluminophosphate molecular sieve catalyst can be continuouslyintroduced as a moving bed to a regeneration zone where it can beregenerated, such as, for example, by removing carbonaceous materials byoxidation in an oxygen-containing atmosphere. In the preferred practiceof the invention, the catalyst will be subject to a regeneration step byburning off carbonaceous deposits accumulated during reactions.

In converting methanol to olefins using the catalyst compositions of theinvention, the process is preferably carried out in the vapor phase suchthat the feedstock is contacted in a vapor phase in a reaction zone witha silicoaluminophosphate molecular sieve at effective process conditionssuch as to produce light olefins, i.e., an effective temperature,pressure, WHSV (weight hourly space velocity) and, optionally, aneffective amount of diluent, correlated to produce light olefins.Alternatively, the process may be carried out in a liquid phase. Whenthe process is carried out in the liquid phase, the process necessarilyinvolves the separation of products formed in a liquid reaction mediaand can result in different conversions and selectivities of feedstockto product with respect to the relative ratios of the light olefinproducts as compared to that formed by the vapor phase process.

The temperatures which may be employed in the process may vary over awide range depending, at least in part, on the selectedsilicoaluminophosphate catalyst. In general, the process can beconducted at an effective temperature between about 200° and about 700°C., preferably between about 250° and about 600° C., and most preferablybetween about 300° and about 500° C. Temperatures outside the statedrange are not excluded from the scope of this invention, although suchdo not fall within certain desirable embodiments of the invention. Atthe lower end of the temperature range and, thus, generally at the lowerrate of reaction, the formation of the desired light olefin products maybecome markedly slow. At the upper end of the temperature range andbeyond, the process may not form an optimum amount of light olefinproducts. Notwithstanding these factors, the reaction will still occurand the feedstock, at least in part, can be converted to the desiredlight olefin products at temperatures outside the range between about200° and about 700° C.

The process is effectively carried out over a wide range of pressuresincluding autogenous pressures. At pressures between about 0.001atmospheres and about 1000 atmospheres, light olefin products will notnecessarily form at all pressures. The preferred pressure is betweenabout 0.01 atmospheres and about 100 atmospheres. The pressures referredto herein for the process are exclusive of the inert diluent, if any ispresent, and refer to the partial pressure of the feedstock as itrelates to methanol. Pressures outside the stated range are not excludedfrom the scope of this invention, although such do not fall withincertain desirable embodiments of the invention. At the lower and upperend of the pressure range, light olefin products can be formed but theprocess will not be optimum.

The process is run for a period of time sufficient to produce thedesired light olefin products. In general, the residence time employedto produce the desired product can vary from seconds to a number ofhours. It will be readily appreciated by one skilled in the art that theresidence time will be determined to a significant extent by thereaction temperature, the silicoaluminophosphate molecular sieveselected, the weight hourly space velocity (WHSV), the phase (liquid orvapor) selected and, perhaps, selected reactor design characteristics.

The process is effectively carried out over a wide range of WHSV for thefeedstock and is generally between about 0.01 and about 100 hr⁻¹ andpreferably between about 0.1 and about 40 hr⁻¹. Values above 100 hr⁻¹may be employed and are intended to be covered by the instant process,although such are not preferred.

The instant process is most preferably carried out under processconditions comprising a temperature between about 300° and about 500°C., a pressure between about 0.1 atmosphere (one atmosphere equals 14.7psia) and about 100 atmospheres, utilizing a WHSV expressed in hr⁻¹ foreach component of the feedstock having a value between about 0.1 andabout 40. The temperature, pressure, and WHSV are each selected suchthat the effective process conditions, i.e., the effective temperature,pressure, and WHSV are employed in conjunction, i.e., correlated, withthe selected silicoaluminophosphate molecular sieve and selectedfeedstock such that light olefin products are produced.

The amount of fresh feedstock fed separately or jointly with a vaporfeedstock, to the reactor system is in the range of from 0.1 to about 85wt-%, preferably from about 1 to about 75 wt-%, more preferably fromabout 5 to about 65 wt-% based on the total weight of the feedstockincluding any diluent contained therein. The liquid and vapor feedstocksare preferably the same composition, or contain varying proportions ofthe same or different feedstock with the same or different diluent.

Molecular sieves of this invention have a roughly rectangularparallelepiped crystal morphology or intergrowth of roughly rectangularparallelepiped crystal morphologies or a mixture thereof. This includescrystals which are cubic in which all the dimensions are the same, butalso those in which the aspect ratio is less than or equal to 5 andpreferably less than or equal to two. It is also necessary that theaverage smallest crystal dimension be at least 50 nanometers andpreferably at least 100 nanometers.

As is explained in the examples, the morphology of the crystals and theaverage smallest crystal dimension is determined by examining the ELAPOmolecular sieve using scanning electron microscopy (SEM) and measuringthe crystals in order to obtain an average value for the smallestdimension.

Without wishing to be bound by any one particular theory, it appearsthat a minimum thickness is required so that the diffusion path fordesorption of ethylene and propylene is sufficiently long to allowdifferentiation of the two molecules. The ELAPOs and more particularly,the silicoaluminophosphates which are synthesized using the processdescribed above will usually contain some of the organic templatingagent in its pores. In order for the ELAPOs to be active catalysts, thetemplating agent in the pores must be removed by heating the ELAPOpowder in an oxygen containing atmosphere at a temperature of about 200°to about 700° C. until the template is removed, usually a few hours.

A preferred embodiment of the invention is one in which the metal (EL)content varies from about 0.005 to about 0.05 mole fraction. Anespecially preferred embodiment is one in which EL is silicon (usuallyreferred to as SAPO). The SAPOs which can be used in the instantinvention are any of those described in U.S. Pat. No. 4,440,871. Of thespecific crystallographic structures described in the '871 patent, theSAPO-34, i.e., structure type 34, is preferred. The SAPO-34 structure ischaracterized in that it adsorbs xenon but does not adsorb isobutane,indicating that it has a pore opening of about 4.2 angstroms. SAPO-34 isa silica-aluminophosphate material with the chabazite (CHA) frameworkstructure. The structure is rhombohedral and can be described as astacking of sheets along <100> directions in the crystal structure. Thesheets contain slanted double six rings. In successive sheets all doublesix rings are slanted in the same direction giving an AAAA stackingsequence along the rhombohedral <100> directions. If the direction ofslant is reversed every second plane, creating an ABABAB stackingsequence, the ALPO-18 structure (AEI) results. Both of these structuretypes consist of the same sheet structure, stacked with a differentsequence, either AAAA (CHA) or ABAB (AEI). This similarity is the basisfor faulting representing a mixture of stacking sequences intermediatebetween these end members. Many SAPO-34 materials show at least somedegree of faulting, as evidenced by broadening of x-ray diffractionpeaks and streaking of reflections in electron diffraction patterns.

By varying the composition, specifically Si/(Al+P), over broad ranges,the degree of faulting can be influenced and there is a trend such thathigh Si/(Al+P) materials show less faulting, all other factors ofsynthesis being equal. This is found for a variety of formulations.However, the degree of faulting is quite sensitive to a number offactors and exceptions to the trend can be expected for differentsyntheses in a narrow range of Si/(Al+P) ratio.

Varying compositions can also influence particle morphology. In generalfor SAPO-34 preparations at greater than 5 wt-% SiO₂ in the finalproduct, the morphology is roughly described as cube-like, whereas mostbut not all materials produced in the range below about 4 wt-% SiO₂ inthe final product tend to have a plate-like morphology. In fact,morphology and degree of faulting were found to be correlated. Thefaults break the rhombohedral symmetry by distinguishing one of theotherwise equivalent <100> planes; crystals grow more rapidly in theunfaulted directions and a plate-like morphology tends to result.

It has been found that the selectivity of the reaction to C₂ and C₃olefins in the MTO reaction process is negatively impacted by largefault densities (as determined by x-ray diffraction of calcined SAPO-34compounds).

It has further been found that a more three-dimensional morphology,being either cube-like or consisting of intersecting plates (with faultsalong distinct <100> planes) is beneficial to selectivity in the MTOreaction.

These factors indicate that a high ratio of Si/(Al+P) would be desirablein SAPO-34 for use in the MTO process. Unfortunately, it has been foundthat increasing the silica level raises the acidity of the catalystwhich may accelerate catalyst sensitivity to deactivation by coking.

A method of producing SAPO-34 catalyst having the desired morphology hasnow been discovered. This method measurably decreases the AEI faultdensity without increasing the overall silicon level. It also increasesthe degree of desirable roughly rectangular parallelepiped crystalmorphology growth as opposed to less desirable plate-like growth andallows the morphology of the crystals to be influenced withoutincreasing the level of silicon in the SAPO-34 crystal.

The key to the method of preparation of the molecular sieves in thepresent invention is the use of seeds that have a relatively highSi/(Al+P) level. The high silicon level in the seeds results in SAPO-34crystals that are relatively fault-free. Without wishing to be bound bya particular theory as to the mechanism involved, the use of the seedsin the synthesis of SAPO-34 results in SAPO-34 having a lower faultdensity than obtained without the use of the seeds. The seeds themselvesare cube-like and present surfaces for nucleation of subsequent growthin all three directions, resulting in a morphology which isthree-dimensional, as opposed to simple cubes. The seeds used in thepresent invention preferably contain from about 5 to 20 wt-% SiO₂, morepreferably contain about 8 to 15 wt-% SiO₂ and most preferably containabout 10 to 15 wt-% SiO₂. The following table shows that when seedscontaining less than 3 wt-% SiO₂ are used, the selectivity to productionof the desired ethylene and propylene was about 81% and when the seedshaving SiO₂ in the desired range was used, the selectivity was about85%.

Selectivity for Seed quantity Seed SiO₂ Sample SiO₂, wt-% ethylene andas wt-% of wt-%, No. volatile free propylene wt-% dry product volatilefree 1 3.2 81 0.4 2.8 2 3.2 84.9 4.0 14.7 3 2.7 84.7 4.0 12.1 4 3.6 84.34.0 9.0An increase of yield in this amount is considered significant inimproving the efficiency and profitability of an MTO plant. Sample 1 isof a prior art material made without use of the seeds having a higherlevel of SiO₂ while Samples 2-4 are samples within the scope of thepresent invention that were prepared in accordance with the methoddescribed in the following Example.

Example

A quantity of catalyst prepared in accordance with the present inventionwas made. In a container, 345 grams of orthophosphoric acid (85%) wascombined with 403 grams of water. To this mixture was added 30 grams ofa silica suspension (Ludox LS, Aldrich Chemicals) and 628 grams of a 35wt-% aqueous solution of tetraethylammonium hydroxide (TEAOH). Then 212grams alumina in the form of pseudo-boehmite, (sold by UOP LLC as Versal250) along with water and 18 grams of SAPO-34 seed material containingabout 9 wt-% silica were added and blended in. In addition, a quantityof catalyst was made without the higher silica seeds that are used inthe present invention for purposes of comparison

The mixtures were then placed in a steel pressure reactor equipped witha turbine stirrer. The mixtures were stirred and heated to 100° C. overa 6-hour period, then held at 100° C. for 9 hours, heated to 175° C.over a period of 5 hours and then held at 175° C. for 48 hours. Finally,the reaction mixture was cooled to ambient room temperature over aperiod of 8 hours. The solid product was recovered by centrifugationover a 20-minute period, and washed with water. A series of threeadditional reslurry, centrifugation, decant steps followed and then theproduct was dried overnight at 95° C. Several characterization methodswere used to examine properties of the product including the faultdensity (X-ray diffraction), morphology (transmission electronmicroscopy and scanning electron microscopy) and chemical analysis(transmission electron microscopy).

Samples 2 and 3 showed X-ray diffraction patterns with relativelywell-formed and sharp peaks indicative of crystallites with the CHAstructure type and only small amounts of AEI faulting. Sample 1,exhibited broadening of some peaks and the appearance of new peaks inthe diffraction pattern, indicating the presence of significant amountsof AEI faulting. Further analysis of the data confirmed theseconclusions.

Scanning electron microscopy showed that the morphology of Samples 2 and3 were consistent with cubes or thick, intergrown plates, while themorphology of Sample 1 was best described as thin plates. Transmissionelectron microscopy confirmed this comparison of the morphology of thesamples.

Further analysis of the samples disclosed that there were often areas ofelevated silicon content, typically in the center of particles, whichare plausibly explained as particles of seed. Material at the edges ofthe particles tended to contain lower levels of silicon.

1. A catalyst comprising a crystalline metallo-aluminophosphatemolecular sieve wherein a first portion of said crystallinemetallo-aluminophosphate molecular sieve comprises about 5 to 20 wt-%silica and wherein a second portion of said crystallinemetallo-aluminophosphate comprises about 1 to 5 wt-% silica.
 2. Thecatalyst of claim 1 wherein a first portion of said crystallinemetallo-aluminophosphate molecular sieve comprises about 8 to 15 wt-%silica and wherein a second portion of said crystallinemetallo-aluminophosphate comprises about 2 to 4 wt-% silica.
 3. Thecatalyst of claim 1 where the crystalline metallo-aluminophosphatemolecular sieve has the crystal structure of SAPO-34.
 4. The catalyst ofclaim 1 comprising crystalline metallo-aluminophosphate molecular sieveand an inorganic oxide binder.
 5. The catalyst of claim 4 where theinorganic oxide binder is selected from the group consisting of alumina,silica, aluminum phosphate, silica-alumina and mixtures thereof.
 6. Thecatalyst of claim 1 where the crystalline metallo-aluminophosphatemolecular sieve is present in an amount from about 10 to about 90 wt-%of the catalyst.
 7. The catalyst of claim 1 where the crystallinemetallo-aluminophosphate molecular sieve exhibits a roughly rectangularparallelepiped crystal morphology or intergrowth of roughly rectangularparallelepiped crystal morphologies or a mixture thereof where theaspect ratio of the dimensions of the crystals largest to smallestdimension is less than or equal to 2.